1. Field of the Invention
The present invention relates to a thermal infrared detecting device (also referred to as “thermal infrared imaging device”) which detects a temperature change generated by an incident infrared ray with two-dimensionally arrayed semiconductor sensors, and more particularly to a thermal infrared detecting device including a diode as a temperature sensor.
2. Related Art
Conventionally, there have been developed various techniques relating to a thermal infrared solid-state imaging device for detecting a temperature change generated by the incident infrared ray with the arrayed semiconductor sensors.
For example, JP-A-2003-110938 discloses a thermal infrared detecting device which applies a constant forward voltage to a diode and utilizes temperature dependence of a current flowing through the diode to detect the temperature change. The constant-voltage drive scheme which applies the constant forward voltage to drive the diode provides a high-sensitivity thermal infrared detecting device. The diode forward current is exponentially increased with respect to the voltage, and therefore a larger rate of change is obtained by detecting change in the forward current when applying the constant forward voltage than by detecting change in the forward voltage when applying the constant forward current. Therefore, the high-sensitivity thermal infrared detecting device can be realized by detecting change in the forward current. JP-A-2000-019015, JP-A-2001-044400 and JP-A-2001-264176 also disclose a similar type of thermal infrared detecting device.
In the conventional thermal infrared detecting device, plural diodes are arrayed as a temperature sensor, each diode is connected to a row selection line and a signal line, and detection result is read from one pixel with the row selection line and the signal line.
In the viewpoint of constant-voltage drive of the diode, the important point is how a predetermined bias is applied to the diode.
Usually, the thermal infrared detecting device using the diode has a hollow heat-insulated structure in which a thermoelectric conversion section is supported by two elongated heat-insulated supporting legs. The diode is incorporated in the thermoelectric conversion section, and interconnection to the diode is embedded in the heat-insulated supporting leg. An infrared absorption section is provided on a top of the thermoelectric conversion section. Infrared ray incident to the infrared absorption section changes infrared energy absorbed by the infrared absorption unit, and the heat-insulated structure converts the change in the infrared energy into change in a temperature of the thermoelectric conversion section. The temperature change is read with the change in current flowing through the diode incorporated in the thermoelectric conversion section. In order to enhance detection sensitivity in the configuration of the thermal infrared detecting device, it is necessary to increase a thermal resistance of the heat-insulated supporting leg. Additionally, a metal constituting the interconnection embedded in the heat-insulated supporting leg is preferably formed into a thin film and thinned and lengthened. Therefore, an electric resistance of the interconnection becomes high such as several kilo ohms to ten and several kilo ohms.
The following problem may be generated when a predetermined bias is applied to the diode under the high electric resistance of the interconnection.
In the case where a predetermined bias voltage is supplied between the interconnection in the supporting leg and a connecting point of a row selection line and a signal line from an external circuit, a temperature is raised at the thermoelectric conversion section to increase the diode current, and a voltage drop is increased at the interconnection in the supporting leg. This causes the diode forward voltage to be decreased, decreasing the current flowing through the diode. On the contrary, when the temperature is lowered at the thermoelectric conversion section to decrease the diode current, the voltage drop is decreased at the interconnection in the supporting leg. This causes the diode forward voltage to be increased, increasing the diode current. Thus, there is phenomenon in which the change in diode current is suppressed by the influence of the voltage drop caused by the interconnection resistance. That is, when the diode temperature is changed to fluctuate the current flowing through the diode, the effective bias voltage applied to the diode is fluctuated due to the fluctuation in voltage drop caused by the diode or a resistance of the interconnection, so that the change in diode current associated with the temperature change is suppressed and temperature detecting sensitivity is lowered. Hereinafter the effect of suppressing the change in diode current by the interconnection resistance is referred to as “negative feedback effect”. The negative feedback effect is caused by a resistance from the point to be applied with the constant voltage to the diode. Although major factor of the resistance is the interconnection resistance in the supporting leg, resistances of the row selection line and signal line largely contribute to the resistance causing the negative feedback effect. The negative feedback effect causes a problem in that a characteristic of high sensitivity as the feature of the constant-voltage drive scheme cannot sufficiently be exerted.
JP-A-2003-110938 describes that, regarding the negative feedback effect, fluctuation in voltage across means for converting current to voltage in order to read the diode current (for example, fluctuation in voltage at load resistance or load capacitance which is connected as the converting means) has an influence on the diode bias. JP-A-2003-110938 discloses a method for solving the negative feedback effect, which uses the voltage converting means to always keep a voltage of a connecting point between the signal line and column transistors composing the voltage converting means, constant. However, the method cannot solve the negative feedback effect caused by the resistances of the signal line and selection line to the column transistors as the voltage converting means and the interconnection resistance in the pixel.
JP-A-2000-019015 discloses an infrared detecting device which utilizes temperature dependence of the diode forward characteristic. Specifically, it discloses that the diode bias is changed with a variable voltage source such that the current change (drift current) is suppressed when ambient temperature is changed. Although JP-A-2000-019015 does not disclose the detailed method for changing the diode bias, it can be understood, for example, that the diode bias is lowered to decrease the current in order to keep the output constant when ambient temperature is raised to increase the diode current. However, in the method, it is clear that the negative feedback effect caused by the interconnection resistance generated in the incidence of the infrared ray cannot be eliminated. Originally, JP-A-2000-019015 does not recognize the problem with the negative feedback effect.
JP-A-2001-044400 relates to a structure for widening a diode contact area, and discloses a configuration of the read circuit similar to that of JP-A-2000-019015. Therefore, similarly to JP-A-2000-019015, JP-A-2001-044400 does not solve the problem of the negative feedback effect caused by the interconnection resistance generated in the incidence of the infrared ray. Originally, JP-A-2001-044400 does not recognize the problem with the negative feedback effect.
Similarly to JP-A-2000-019015, JP-A-2001-264176 discloses a temperature measuring device or a thermal infrared image sensor which has a bias voltage circuit inserted serially to the diode to read the forward current by the bias voltage circuit. JP-A-2001-264176 describes the negative feedback effect caused by the resistance, and points out that the negative feedback effect becomes troublesome when a resistor connected for reading the current and raising the output is increased. JP-A-2001-264176 describes that the accurate bias is supplied to the diode irrespective of the resistance of the current read section based on the same recognition about the problem as JP-A-2003-110938. In configuration of JP-A-2001-264176, when a diode with heat-insulated structure is applied to the thermal infrared sensor, since a bias circuit is usually formed in a substrate having no heat-insulated structure, additional interconnection to the bias circuit from the interconnection in the supporting leg in the heat-insulated structure is required, which generates the negative feedback effect.